A non-anhydrous, minimally basic protocol for the simplification of nucleophilic 18F-fluorination chemistry

[18F]fluoride Activation and 18F-Labelling in Hydrous Conditions-Towards a Microfluidic Synthesis of PET Radiopharmaceuticals

18F-labelled radiopharmaceuticals are indispensable in positron emission tomography. The critical step in the preparation of 18F-labelled tracers is the anhydrous F-18 nucleophilic substitution reaction, which involves [18F]F- anions generated in aqueous media by the cyclotron. For this, azeotropic drying by distillation is widely used in standard synthesisers, but microfluidic systems are often not compatible with such a process. To avoid this step, several methods compatible with aqueous media have been developed. We summarised the existing approaches and two of them have been studied in detail. [18F]fluoride elution efficiencies have been investigated under different conditions showing high 18F-recovery. Finally, a large scope of precursors has been assessed for radiochemical conversion, and these hydrous labelling techniques have shown their potential for tracer production using a microfluidic approach, more particularly compatible with iMiDEV™ cassette volumes.

CycloSiFA: The Next Generation of Silicon-Based Fluoride Acceptors for Positron Emission Tomography (PET)

The ring-opening Si-fluorination of a variety of azasilole derivatives cyclo-1-(iPr2 Si)-4-X-C6 H3 -2-CH2 NR (4: R=2,6-iPr2 C6 H3 , X=H; 4 a: R=2,4,6-Me3 C6 H2 , X=H; 9: R=2,6-iPr2 C6 H3 , X=tBuMe2 SiO; 10: R=2,6-iPr2 C6 H3 , X=OH; 13: R=2,6-iPr2 C6 H3 , X=HCCCH2 O; 22: R=2,6-iPr2 C6 H3 , X=tBuMe2 SiCH2 O) with different 19 F-fluoride sources was studied, optimized and the experience gained was used in a translational approach to create a straightforward 18 F-labelling protocol for the azasilole derivatives [18 F]6 and [18 F]14. The latter constitutes a potential clickable CycloSiFA prosthetic group which might be used in PET tracer development using Cu-catalysed triazole formation. Based on our findings, CycloSiFA has the potential to become a new entry into non-canonical labelling methodologies for radioactive PET tracer development.

[18F]Tosyl fluoride as a versatile [18F]fluoride source for the preparation of 18F-labeled radiopharmaceuticals

Positron emission tomography (PET) is an in vivo imaging technology that utilizes positron-emitting radioisotope-labeled compounds as PET radiotracers that are commonly used in clinic and in various research areas, including oncology, cardiology, and neurology. Fluorine-18 is the most widely used PET-radionuclide and commonly produced by proton bombardment of ¹⁸O-enriched water in a cyclotron. The [¹⁸F]fluoride thus obtained generally requires processing by azeotropic drying in order to completely remove H₂O before it can be used for nucleophilic radiofluorination. In general, the drying step is important in facilitating the radiofluorination reactions and the preparation of ¹⁸F-labeled PET radiotracers. In this communication, we have demonstrated the feasibility of using [¹⁸F]tosyl fluoride ([¹⁸F]TsF) as a versatile [¹⁸F]fluoride source for radiofluorination to bypass the azeotropic drying step, and we have developed a continuous flow solid-phase radiosynthesis strategy to generate [¹⁸F]TsF in a form that is excellent for radiofluorination. [¹⁸F]TsF shows high reactivity in radiofluorination and provides the features suitable for preparing PET radiotracers on a small scale and exploring novel radiolabeling technologies. Thus, using [¹⁸F]TsF as a [¹⁸F]fluoride source is a promising strategy that facilitates radiofluorination and provides a convenient and efficient solution for the preparation of ¹⁸F-labeled radiopharmaceuticals that is well matched to the emerging trends in PET imaging technologies.

Copper-Mediated Radiobromination of (Hetero)Aryl Boronic Pinacol Esters

A copper-mediated radiobromination of (hetero)aryl boronic pinacol esters is described. Cyclotron-produced [76/77Br]bromide was isolated using an anion exchange cartridge, wherein the pre-equilibration and elution solutions played a critical role in downstream deboro-bromination. The bromination tolerates a broad range of functional groups, labeling molecules with ranging electronic and steric effects. Bologically active radiopharmaceuticals were synthesized, including two radiobrominated inhibitors of poly ADP ribose polymerase, a clinically relevant chemotherapeutic target for ovarian, breast, and prostate cancers.

Detrimental impact of aqueous mobile phases on 18F-labelled radiopharmaceutical analysis via radio-TLC

The list of new positron-emission tomography (PET) tracers has rapidly grown in the past decade, following discoveries of new biological targets and therapeutic strategies, with several compounds garnering recent regulatory approval for clinical use. During the development of synthesis methods and production of new tracers for imaging, analytical methods for radio-high performance liquid chromatography (radio-HPLC) and radio-thin layer chromatography (radio-TLC) separations need to be developed to assess radiochemical compositions. Radio-TLC is often faster, simpler, and sometimes more accurate than radio-HPLC (as there is no underestimation of [18F]fluoride when analyzing 18F-labeled radiopharmaceuticals). Many protocols have been developed for separating 18F-radiopharmaceuticals on silica TLC plates, typically with [18F]fluoride retained at the origin and the radiopharmaceutical (and impurities) migrating along the plate. Interestingly, many reports describe the use of aqueous conditions to mobilize polar species, but it is known that aqueous conditions can modify silica and alter its chromatographic behavior. In this technical note, we explore the effects that aqueous conditions have on the analysis of 18F-radiopharmaceutical mixtures, revealing that with sufficient water, the radionuclide ([18F]fluoride) can migrate away from the origin and can be split into multiple bands. Furthermore, water can hinder the migration of the radiopharmaceutical. These effects can lead to overlapped bands or reversal of the normally expected order of bands, potentially leading to the misinterpretation of results if care is not taken to validate the TLC method carefully.

A rapid and systematic approach for the optimization of radio thin-layer chromatography resolution

Radiopharmaceutical analysis is limited by conventional methods. Radio-HPLC may be inaccurate for some compounds (e.g., 18F-radiopharmaceuticals) due to radionuclide sequester. Radio-TLC is simpler, faster, and detects all species but has limited resolution. Imaging-based readout of TLC plates (e.g., using Cerenkov luminescence imaging) can improve readout resolution, but the underlying chromatographic separation efficiency may be insufficient to resolve chemically similar species such as product and precursor-derived impurities. This study applies a systematic mobile phase optimization method, PRISMA, to improve radio-TLC resolution. The PRISMA method optimizes the mobile phase by selecting the correct solvent, optimizing solvent polarity, and optimizing composition. Without prior knowledge of impurities and by simply observing the separation resolution between a radiopharmaceutical and its nearest radioactive or non-radioactive impurities (observed via UV imaging) for different mobile phases, the PRISMA method enabled the development of high-resolution separation conditions for a wide range of 18F-radiopharmaceuticals ( [18F]PBR-06, [18F]FEPPA, [18F]Fallypride, [18F]FPEB, and [18F]FDOPA). Each optimization required a single batch of crude radiopharmaceutical and a few hours. Interestingly, the optimized TLC method provided greater accuracy (compared to other published TLC methods) in determining the product abundance of one radiopharmaceutical studied in more depth ( [18F]Fallypride) and was capable of resolving a comparable number of species as isocratic radio-HPLC. We used the PRISMA-optimized mobile phase for [18F]FPEB in combination with multi-lane radio-TLC techniques to evaluate reaction performance during high-throughput synthesis optimization of [18F]FPEB. The PRISMA methodology, in combination with high-resolution radio-TLC readout, enables a rapid and systematic approach to achieving high-resolution and accurate analysis of radiopharmaceuticals without the need for radio-HPLC.

Nucleophilic Radiofluorination Using Tri-tert-Butanol Ammonium as a Bifunctional Organocatalyst: Mechanism and Energetics

We present a quantum chemical analysis of the ¹⁸F-fluorination of 1,3-ditosylpropane, promoted by a quaternary ammonium salt (tri-(tert-butanol)-methylammonium iodide (TBMA-I) with moderate to good radiochemical yields (RCYs), experimentally observed by Shinde et al. We obtained the mechanism of the S_N2 process, focusing on the role of the –OH functional groups facilitating the reactions. We found that the counter-cation TBMA⁺ acts as a bifunctional promoter: the –OH groups function as a bidentate ‘anchor’ bridging the nucleophile [¹⁸F]F⁻ and the –OTs leaving group or the third –OH. These electrostatic interactions cooperate for the formation of the transition states of a very compact configuration for facile S_N2 ¹⁸F-fluorination.

SNAr Radiofluorination with In Situ Generated [18F]Tetramethylammonium Fluoride

This report describes a method for the nucleophilic radiofluorination of (hetero)aryl chlorides, (hetero)aryl triflates, and nitroarenes using a combination of [¹⁸F]KF·K_2.2.2 and Me₄NHCO₃ for the in situ formation of a strongly nucleophilic fluorinating reagent (proposed to be [¹⁸F]Me₄NF). This method is applied to 24 substrates bearing diverse functional groups, and it generates [¹⁸F](hetero)aryl fluoride products in good to excellent radiochemical yields in the presence of ambient air/moisture. The reaction is applied to the preparation of ¹⁸F-labeled HQ-415 for potential (pre)clinical use.

The Non-Anhydrous, Minimally Basic Synthesis of the Dopamine D2 Agonist [18F]MCL-524

The dopamine D2 agonist MCL-524 is selective for the D2 receptor in the high-affinity state (D2high), and, therefore, the PET analogue, [18F]MCL-524, may facilitate the elucidation of the role of D2high in disorders such as schizophrenia. However, the previously reported synthesis of [18F]MCL-524 proved difficult to replicate and was lacking experimental details. We therefore developed a new synthesis of [18F]MCL-524 using a "non-anhydrous, minimally basic" (NAMB) approach. In this method, [18F]F- is eluted from a small (10-12 mg) trap-and-release column with tetraethylammonium tosylate (2.37 mg) in 7:3 MeCN:H2O (0.1 mL), rather than the basic carbonate or bicarbonate solution that is most often used for [18F]F- recovery. The tosylated precursor (1 mg) in 0.9 mL anhydrous acetonitrile was added directly to the eluate, without azeotropic drying, and the solution was heated (150 °C/15 min). The catechol was then deprotected with the Lewis acid In(OTf)3 (10 equiv.; 150 °C/20 min). In contrast to deprotection with protic acids, Lewis-acid-based deprotection facilitated the efficient removal of byproducts by HPLC and eliminated the need for SPE extraction prior to HPLC purification. Using the NAMB approach, [18F]MCL-524 was obtained in 5-9% RCY (decay-corrected, n = 3), confirming the utility of this improved method for the multistep synthesis of [18F]MCL-524 and suggesting that it may applicable to the synthesis of other 18F-labeled radiotracers.

18F-Fluorination Using Tri-Tert-Butanol Ammonium Iodide as Phase-Transfer Catalyst: An Alternative Minimalist Approach

The ¹⁸F syntheses of tracers for positron emission tomography (PET) typically require several steps, including extraction of [¹⁸F]fluoride from H₂[¹⁸O]O, elution, and drying, prior to nucleophilic substitution reaction, being a laborious and time-consuming process. The elution of [¹⁸F]fluoride is commonly achieved by phase transfer catalysts (PTC) in aqueous solution, which makes azeotropic drying indispensable. The ideal PTC is characterized by a slightly basic nature, its capacity to elute [¹⁸F]fluoride with anhydrous solvents, and its efficient complex formation with [¹⁸F]fluoride during subsequent labeling. Herein, we developed tri-(tert-butanol)-methylammonium iodide (TBMA-I), a quaternary ammonium salt serving as the PTC for ¹⁸F-fluorination reactions. The favorable elution efficiency of [¹⁸F]fluoride using TBMA-I was demonstrated with aprotic and protic solvents, maintaining high ¹⁸F-recoveries of 96-99%. ¹⁸F-labeling reactions using TBMA-I as PTC were studied with aliphatic 1,3-ditosylpropane and aryl pinacol boronate esters as precursors, providing ¹⁸F-labeled products in moderate-to-high radiochemical yields. TBMA-I revealed adequate properties for application to ¹⁸F-fluorination reactions and could be used for elution of [¹⁸F]fluoride with MeOH, omitting an additional base and azeotropic drying prior to ¹⁸F-labeling. We speculate that the tert-alcohol functionality of TBMA-I promotes intermolecular hydrogen bonding, which enhances the elution efficiency and stability of [¹⁸F]fluoride during nucleophilic ¹⁸F-fluorination.

Scalable 18F processing conditions for copper-mediated radiofluorination chemistry facilitate DoE optimization studies and afford an improved synthesis of [18F]olaparib

A convenient and scalable base-free method for processing [¹⁸F]fluoride as [¹⁸F]TBAF is reported and applied to copper-mediated radiofluorination radiosyntheses. A central feature of this method is that a single production of [¹⁸F]TBAF can be divided into small aliquots that can be used to perform multiple small-scale reactions in DoE optimization studies. The results of these studies can then be reliably translated to full batch tracer productions using automated synthesizers. The processing method was applied to the DoE optimization of [¹⁸F]olaparib, affording the tracer in high radiochemical yields via both manual (%RCY = 78 ± 6%, n = 4 (CMRF step only)) and automated (up to 80% (%RCY); 41% activity yield (%AY)) radiosynthesis procedures.

Novel late-stage radiosynthesis of 5-[18F]-trifluoromethyl-1,2,4-oxadiazole (TFMO) containing molecules for PET imaging

Small molecules that contain the (TFMO) moiety were reported to specifically inhibit the class-IIa histone deacetylases (HDACs), an important target in cancer and the disorders of the central nervous system (CNS). However, radiolabeling methods to incorporate the [18F]fluoride into the TFMO moiety are lacking. Herein, we report a novel late-stage incorporation of [18F]fluoride into the TFMO moiety in a single radiochemical step. In this approach the bromodifluoromethyl-1,2,4-oxadiazole was converted into [18F]TFMO via no-carrier-added bromine-[18F]fluoride exchange in a single step, thus producing the PET tracers with acceptable radiochemical yield (3–5%), high radiochemical purity (> 98%) and moderate molar activity of 0.33–0.49 GBq/umol (8.9–13.4 mCi/umol). We validated the utility of the novel radiochemical design by the radiosynthesis of [18F]TMP195, which is a known TFMO containing potent inhibitor of class-IIa HDACs.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board (January-June 2020)

BACKGROUND

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to describe trends in the field.

RESULTS

This commentary of highlights has resulted in 19 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals.

CONCLUSION

Trends in radiochemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

Automated synthesis of the 16α-[18F]fluoroestradiol ([18F]FES): minimization of precursor amount and resulting benefits

The 16α-[18F]Fluoroestradiol ([18F]FES) is an established PET radiotracer for estrogen positive (ER+) breast cancer. Although the radiosynthesis is well-described, the majority of the published methods suffer from modest or irreproducible yields and time-intensive purification procedures. In view of the considerable clinical applications, development of a more efficient and faster synthesis of [18F]FES still remains a task of a significant practical importance. [18F]FES was produced by a direct nucleophilic radiofluorination of 3-O-methoxymethyl-16,17-O-sulfuryl-16-epiestriol (MMSE), followed by acidic hydrolysis using HCl/CH3CN. [18F]Fluoride retained on a QMA carb cartridge (46 mg) was eluted by solution of 1.2 mg of tetrabutylammonium tosylate (TBAOTs) in EtOH. After fluorination reaction (0.3 mg MMSE, 1 ml of CH3CN/100 °C, 5 min) [18F]FES was isolated by single-cartridge SPE purification using OASIS WAX 3cc, elution accomplished with aqueous ethanol of different concentrations. On а GE TRACERlab FX N Pro automated module [18F]FES (formulated in normal saline with 5% EtOH) was obtained in 33 ± 3% yield (n = 5, non-decay corrected) within 32 min. Reduction of precursor amount, exclusion of azeotropic drying step and simplification of purification make the suggested method readily adaptable to various automated synthesizers and offers significant cost decrease.